Why 917?

[Lucien]

I have a question for the modding guys:

There's been quite a few regulated LED mods, where the drive current is 917mA. Why this particular number? /ubbthreads/images/graemlins/icon3.gif

Is it some optimal drive level? Some thermal/current limit?

Or is it the flashaholic equivalent of 42?

Anyone? /ubbthreads/images/graemlins/smile.gif

 

[kj]

I believe that's because McGizmo loves that number /ubbthreads/images/graemlins/smile.gif See this thread.

 

[TheFire]

Well, and it's one that the resistor combinations come out to easily. If you asked for 918, it would be impossible or nearly so, because the current is dependent on the total sense resistance, and that is usually achieved by putting several fractional ohm surface mount resistors in parallel.

 

[McGizmo]

What kj and the fire said. In addition, the actual current to the LED will vary depending on variations and tolerances of the components. With the first 3 1000 mA DownBoys I built, I measured current to the LED all in excess of 1000 mA. I wanted to stay at or below spec and the 917 seemed to be the ticket.

 

[kakster]

1000 mA also seems to be right on the raggedy edge for a luxeon 3/PRT combo when it comes to thermal management. I managed to toast one while doing a runtime test (left it on desk, didnt hold it in my hand). The extra brightness going from ~900mA to 1000mA doesnt seem to be worth the trade off when it comes to the extra heat it generates.

 

[Lucien]

Ah, that all makes a lot of sense. /ubbthreads/images/graemlins/smile.gif

And I definitely agree about the diminishing returns up around the 1 A mark. I got a chance to play with a variable constant current circuit and a Lux III, and I couldn't even eyeball a change in brightness above 1 A.